Generally, a secondary battery refers to a rechargeable battery, while a primary battery refers to a non-rechargeable battery. Secondary batteries are widely used for electronic devices such as cellular phones, notebook computers, video cameras, electric vehicles, or the like. In particular, a lithium secondary battery has an operating voltage of about 3.6 V, triple the capacity of nickel-cadmium batteries or nickel hydrogen batteries generally used as power sources of electronic devices, and due to its high energy density per unit weight, are being utilized more and more.
The lithium secondary battery generally uses lithium oxide and carbonaceous material as a cathode active material and an anode active material, respectively. The lithium secondary battery includes an electrode assembly in which a cathode plate and an anode plate respectively coated with the cathode active material and the anode active material are disposed with a separator interposed between them, and a battery case (an exterior material) which seals and receives the electrode assembly together with an electrolyte.
Meanwhile, depending on the shape of the battery case, a lithium secondary battery may be classified into a can type secondary battery where the electrode assembly is included in a metal can and a pouch type battery where the electrode assembly is included in a pouch of an aluminum laminate. In addition, depending on the shape of the metal can, the can type secondary battery may be further classified into a cylindrical battery and a rectangular battery. The exterior material of the rectangular or cylindrical secondary battery includes a battery case having an open end and a cap assembly sealably coupled to the open end of the battery case.
FIG. 1 is a cross-sectional view schematically showing a general cylindrical secondary battery having a beading portion formed at a battery case.
Referring to FIG. 1, the cylindrical secondary battery generally includes a cylindrical battery case 20 having a sealed bottom and an open top, a jelly-roll type electrode assembly 30 received in the battery case 20, a cap assembly 10 coupled to the upper portion of the battery case 20, a beading portion 40 provided at the front end of the battery case 20 for mounting the cap assembly 10, and a clamping portion 50 for sealing the battery.
The electrode assembly 30 is generally rolled in a jelly-roll shape in a state where a separator is interposed between a cathode plate and an anode plate. A cathode lead 31 is attached to the cathode plate and connected to the cap assembly 10, and an anode lead 32 is attached to the anode plate and connected to the lower end of the battery case 20.
The cap assembly 10 includes a top cap 11 forming a cathode terminal, a safety vent 12 for interrupting a current and/or for exhausting gas when the pressure in the battery increases, an insulation member 13 for electrically separating the safety vent 12 except for a specific portion from a current interrupt member 14, and a current interrupt member 14 connected to a cathode lead 31 connected to the cathode plate, which are laminated in the above order. In addition, the cap assembly 10 is mounted to the beading portion 40 of the battery case 20 in a state of being mounted to a gasket 15. Therefore, in a normal operating condition, the cathode of the electrode assembly 30 is electrically connected to the top cap 11 via the cathode lead 31, the current interrupt member 14 and the safety vent 12.
However, in the general secondary battery having a beading portion as described above, the beading portion 40 should be formed at the battery case 20 in order to couple and fix the cap assembly 10. However, since the accommodation capacity of the electrode assembly 30 decreases as much as the area occupied by the beading portion 40, the battery capacity also decreases. Therefore, in order to solve this problem, a secondary battery having no beading portion 40 formed at the battery case 20 has been recently proposed.
FIG. 2 is a cross-sectional view schematically showing a general cylindrical secondary battery having no beading portion formed at the battery case 20.
Referring to FIG. 2, a beading portion is not formed at the battery case 20. In addition, the cap assembly 10 is coupled to the top of the battery case 20 by means of laser welding or the like, as indicated by L, to seal the battery case 20. At this time, in order to weld the cap assembly 10 to the battery case 20, a cover 16 is generally provided at the outermost portion of the cap assembly 10 as shown in FIG. 2. Since the cover 16 is to be welded to the battery case 20, the cover 16 may be made of a material which can be welded to the battery case 20, for example a metallic material.
In the general secondary battery having no beading portion, a process for welding the cap assembly 10 to the battery case 20 is performed. However, in the welding process, welding segments may be generated, and these welding segments may be adhered to the cap assembly 10 or the electrode assembly 30 and cause an electric short in the battery.
In addition, when the cap assembly 10 is welded to the battery case 20, heat may be generated, and the generated heat may damage various components of the cap assembly 10. Moreover, since the cover 16 made of metal or the like to be welded to the battery case 20 should be separately prepared at the cap assembly 10, the structure of the cap assembly 10 becomes more complicated and increases in manufacturing costs, causing the secondary battery to have a greater weight.